189
ine carbonate breccia beds-a depositional model for two-layer, sediment gravity
o w from the Sekwi Formation (Lower Cambrian), Mackenzie Mountains,
Northwest Territories, Canada
F. F . KRAUSEA N D A . E. OLDERSHAW
I)~,portrm,rltc!f'GcoIo~y. Urli~.rrsityc!f'Crrl,<*rrr:\..Ct11gtrr.v. Altcc.. Crrr~rrtltrT2N IN4
Received September 5. 1978
Revision accepted Septemher 22. 1978
i n the Sekwi Formation. c x b o n a t e breccia beds interhedded with slope sediments are inter-
n single
They are intermediate between true s l u m p and turbidites and may initiate by
\I~~nip~ng;~nvwhere
on the continental slope and rise. Textural characteristics of the deposits are
;I limc~ion
ofttowndope transport distance.
p ~ c ~ c; I d\ \uhnl:~rinesediment gravity flow\ that formed a two-layer deposit during
I I . , I I \ ~ Vevent.
I
Ihm 1;1 formation de Sekwi, on interprete les c o w h e \ de carbonates hl-echique\ interstratifies
dc\ \idinlent\ de talus comme des coulees par gravite d e sediments sousmarins qui ont
I r h ~ r ~ l(In
C &pOt hicouche durant un seul Cpiwcle d e tran\pol-t. Ce\ depbts sont intel-mediaires
cnt~cIc. tltip31s de glissement veritable.; et leh t ~ ~ r h i d i t et
e s peuvent avoir leur origine dans un
cl~\\elncntn'importe ou sur le talus ou la pente continentale. Les caracteristique\ texturales des
dcp111\\011t
kmction de la distance de transport en avnl.
[Traduit par le journal]
.,\N
( . ~ nI I:.~rrhSci..16. 1x9-199(197Y)
Introduction
khjtcli\e 01' this paper is threefold. Firstly,
bide i i tlewiption of carbonate breccia beds
cur within the Lower Cambrian Sekwi For:\esi)ndl?,lo offer an interpretation of the
of rhc\e deposits: and finally, to propose a
I Jepo4rion;d model for submarine carbonI.c!wcr Cambrian Slope Sediments
&dirncnl;ur! rocks of the Lower Cambrian
hrrn;~rit)n,which crop out in the western
nrr;d hl;denzie Mountains of Canada (Fig.
ne;i \hclflo dope transition (Fritz l976ri. h.
:Kr;iu\e ; ~ n dOldershaw 1077. 1978).
\edimtlntsoftheSekwi Formation (Figs. 2,
min;~lcd;ind bedded siltstones, shales, and
~rne\toncs.Typical sedimentary features of
Jc.po\i~$;urc gnulied bedding. bioturbation,
m n t tleform:ution structures that include
iIJing ;und hulting, load casting. convolute
ion, rot;~tetllimestone nodules, and injeccrorc'\. Within this fine-grained sequence
~ i thrth
i have been recognized, 35 of which
ac'~cri\ticallytwo-layer deposits. These
Jrri~cdI'rom the adjacent carbonate shelf
urrountling $lope. flowed downslope, and
&p,\itedon topofadjacent slope sediments.
Breccia Reds
u\c 01 tlic term breccia bed in this paper
I
follows the example of several wol-kers, among
them Cook ct (11. (1972) and Hopkins (1977). It is
clear that the descriptive usefulness of the term
precludes its restriction solely to deposits that
originate as sediment gravity flows. However. for
the purposes of this paper and in the absence of a
better descriptor, a breccia bed is the product of
resedimentation mechanisms and possesses the
attributes illustrated.
Bctlriing Stylr
Breccia beds of the Sekwi Formation typically
are 0.3-5 m thick and laterally extensive. Separate
flows have been traced laterally for 4 km, and may
display marked changes in thickness, pinching, and
swelling at irregular intervals (Fig. 31). Bed thickness commonly doubles at swells. although increases of 4 times have also been observed. In two
places. where a breccia bed was observed to abut
against slope sediments. contacts were blunt and
steep. Breccia beds typically exhibit basal unconformable contacts with scouring and commonly
deformation of underlying lithologies.
B r e c c i ~C1rist.s
~
The most striking chal-acteristic of breccia beds
is their fragmented clastic texture. Breccia bed
clasts are 1-50cm in size and may be shaped
like irregular, indented, angular to subrounded
shingles. 01- as irregular. oblong, and spheroidal
fragments. Dominant clast lithology is calcareous
mudstone, wackestone, and packstone with cross-
OOO8-4O77/79/010IX9-1 I$Ol.(N/O
1979 N;~tion;~l
Research Council of CanadalConseil national de recherche\ du Canada
C A N . J . EARTH SCI. VOL. 16, 1979
I
FIG. I . Index map of Canada with location of study area. Outlined in black are the Lower Cambrian sedimentse\am~ncd>
part of this study. Dashed line indicates position of stl-utigmphic cross section displayed in Fig. 2.
ward progression: massive. poorly graded ,)r *
graded; plane parallel laminated; rippleid
cross-laminated. These units are identical tvtki.
B, and C intervals of the Bouma sequence, ix
contact between breccia and turbidite interrhr
commonly scoured, but unscoured transitionrht
been observed. Scours up to 10m wide ma! n
completely through the underlying breccia. S:?
circular pockets up to 3 m wide and I rn hiyh. v;f
material identical to the grainstones in inrenil,
may occur within and below the base of b r m
interval X.
Internrr/ S ~ ~ N C I I I ~ C
Clasts may be arranged in completely disor- Breccia Bed Seqrrences
Following Walker's (1975, 1976) scheme
ganized, chaotic, and almost random fabrics, or
they may be found in imbricated, aligned, graded, sedimented conglomerates (see Table 1 fo1.a..
or inverse to normally graded arrangements with parison between breccia beds and resedim
coarse tail grading (Fig. 3 e - h ) . Also, breccia beds conglomerates), 3 types of breccia beds have
may be capped by a turbidite which consists of identified in the field. Type 1 beds (Fig. 6)are\
comminuted breccia material (Fig. 3 h. f , g ) . Pres- layered and possess clasts in completel!~J'
ence o r absence of this turbidite layer defines two ganized and chaotically oriented fabric\. CI
broad types: beds that possess only a coarse clastic may be matrix o r grain supported and art
interval (X) and beds that possess both a basal veloped by silty shale. Types 2 and 3 are do
clastic interval (X) and a top turbidite interval ( Y ) layered breccia beds and may be differenti:it
(Figs. 3.4). Interval X comprises clasts in grain and the arrangement of clasts of interval X. Clac
mud support with a matrix of silt, shale, and also type 2 beds (Fig. 6) are disorganized. chao
comminuted breccia material. Typically a breccia oriented, grain supported. and most often pm
(interval X) is overlain by grainstones (interval Y ) silty shale or mixed silty shale and commi
of finely fragmented breccia material. A complete breccia matrix. Type 3 beds have clasts that
Y interval displays grainstones in the following up- normal grading and inverse to no~malgrading
laminated silt to coarse-grained quartz sand. Locally, rare shelf derived clasts consisting of oolitic
grainstone and archeocyat hid boundstone are also
found. Lithology of the majority of clasts is identical with that of fine-grained sediments of the
neighbouring slope from which clasts were derived
(Fig. 3c). Early lithification of slope sediments is
indicated since clasts are fragmented and comminuted. Individual clasts are broken by wedgeshaped cracks and clast edges are commonly angular (Fig. 3 4 .
I
K R A U S E AND OLDERSHAW
191
i ~ r ,1 %hcmatic .;tratigraphic cmss section through the Sekwi Formation. Sections 5. I . 6. 7, 8. 9 show informal units
\ - I : I ~ I I hrpl;~! ;I \hclf:rnd slope transition located by a dashed line on Fig. I. Units A-E are characterized respectively by
upr;ir~d:dtl~ilo\rone\.
cuhtidal cnlcal-eous sedimentary rocks. oolitic carbonates. subtidal shaly silts, and nearshore and
.4d
pl.1111r;~~itl\lone.;
(Krause and Oldershaw 1977. 1978). Biostratigraphic framework is based on a detailed analysis of
I : I I I C I I ~ ~ , I I I I ~l.'ritz(197?,
;I\~!
1973. 1976fr. 1978). Breccia bed occurrences are indicated by x.
positions (Fig. 2). Different breccia types are reconstructed into a lateral depositional model
elsewhere in this paper.
at urrwv
Breccia beds in the Sekwi Formation have been
BII'CC~~Ihell occurrences in a transect across the recognized at localities isolated from each other by
rrgoK~l+trike of the Sekwi Formation are indi- 200 km. They also occur over a distance of 50 km in
M1in Fig. 2 and are also tabulated in Table 2. traverses across strike (Fig. 2).
we-laher breccia heds are restricted to upperosl hrccci;~occurrences at sections 8 and 9 and
Two-layer Sediment Gravity Flow Deposits
at~kcinlyhluxia bed type observed at section 6.
Submarine carbonate breccia beds with a caph n ~ e e ntlouhle-layer type 2 breccia beds (Fig.
H, aver;~giny 30 cm in thickness, were recorded ping turbidite are not unique to the Sekwi Forma'
h e r y e ! heds at section 7; a single type 2 bed tion. Table 3, although not exhaustive, presents
lornoled herwen type 3 beds at this section. One references and comments on submarine carbonate
9 p ! hd. hove type 3 and below single-layer breccia beds discussed in the literature and estabblr, ua$ rccortled at section 9. Type 3 breccia lishes their commonplace occurrence. The exisbbrchar;~crcri\ric;\llyare the lowermost breccia tence of sediment gravity flows in which a basal,
;bl~peencuuntered
at sections 7-9. Important to dense, faster moving flow, with grains supported by
bere iq that wctions 7-9 are in margin to slope dispersive pressure, was overlain by a turbidity
WI\ c o ~ i \ ~ \ tpredominantly
\
of grainstone brecadnddwof d t y \hale with comminuted breccia.
*
192
C A N . J . EARTH SCI.
VOL.. 16.
1979
TABLEI. Main characteristics of breccia beds* a n d resedimented conglomeratest
Breccia beds
Resedimented conglonie
I. Semiconsolidated sediment, supplied by slumping and
sliding of shelf edge a n d slope deposits. Resedimentation takes place downslope from slide
2. Clast lithologies mirror shelf edge and slope sources
3. Breccia bed material builds its own 'channel'
4. An ideal breccia bed consists of two concurrently
deposited layers, a lower clast interval (layer X ) and
an upper turbidite interval (layer Y ) . The turbidite may
contain A, B, and C beds of the Bouma model
5. Three types of breccia beds have been recognized in the
field :
Type 1. Consists of a disorganized clast interval
without a turbidite cap
Type 2. Contains a disorganized basal clast layer and
is capped by a turbiditc
Type 3. The c l a m of layer X display normal o r
inversc t o normal grading and are capped by the
turbidite of layer Y
6. A downslope facies gradation from slumped deposits,
to disorganized breccia beds, t o nornial and inversenormal graded breccia beds is suggested a n d may be a
function of transport distance
I. Unconsolidated sediment pile accumu~ares~n vrillns
watel- a n d is supplied t o canyon feeder channel don4
low stands of sea level and (or) by strong ciirrenrrml
narrow shelf
2. Clast lithologies reflect continental and marine wum
3. Material travels downslope in pre-existing chunneli
4. A resedimented conglon~erateis not usually capped h.2
turbidite interval. Sedimcntary t e x t ~ ~ r throu~h~u~tg
es
bed may superficially resemble the Bouma vyuenx
5. Four models of resedimented conglomerates haw 'm
proposed :
Model I. Disorganized bed
Model 2. Beds where clasts are inverse to normal!!
graded
Model 3. Bed with clasts that are normall!. gradd
Model 4. Deposits where clasts are graded and ka
material may be stratified towards the top. This
model may superficially resemble the Bounia qm.
as both contain a lower massive or graded division*
pass upward into horizontal stratification
6. Models 2, 3, and 4 may reflect a downchannel hcies
gradation and may indicate increasing distance ci
transport
*This papcr and literature on modern dcbris flows, in particular Johnson (1970).
?After Wnlker (1975, 1976, 1978).
TABLE2. Breccia bed occurrence$, Sekwi Formation
umber of flows Single-layer
recorded
Rows
Section 6
Section 7
Section 8
Section 9
Total
4
28
9
6
47
4
0
5
3
12
Double-layer
flows
o
28
4
3
35
current. was strongly advocated by Sanders (1965).
The occurrence of a flowing-grain layer at the base
of a depositing, high density turbidity current was
strikingly confirmed by Middleton (1967). His experiments showed u denser basal flow that moved
downcurrent by shear of the bed, rather than by
rolling and sliding of individual particles. Middle-
ton (1969) proposed that a large scale. high v e l q
grain flow must produce an entrained Iigsrinh
overlying watel- and cause it to flow ~ipidl!.k
suggesttd further that this entrained layer cq
scour the top of the underlying bed (grainf l o ~ l d
rework it. More recently. Hampton (19'21 ba
demonstrated the development of a turbidit!
rent on top of a moving subaqueous dehl-isfloui.
Development of a Two-layer Deposit
The origin of double-layer beds may be u#
stood in terms of the modelling experimen
subaqueous debris flows performed by Ham
(1972) (see Fig. 5 ) . He demonstrated that the
of a subaqueous debris flow deformed in ~ q u n
stresses imposed by the surrounding w t e r .
ment was eroded along the front of the dehri
FIG.3. ( 1 1 ) Breccia bed exposed on north facing cirque wall within d e e p w a t e r sediments of Sekwi Formation. Br
expands and narrows at irregular intervals. A and B arrows point respectively to 'tail' and 'wave' of hreccia bed. Sir&
fentt~reshave been observed with subaerial debris flows and also in flume experiments with subaqueous flow\. .ArmuC
pointy to scoured base of breccia bed. Thickness of deposit at 'wave' approximately I? m. Section 7. ( h )Breccia hsd1.!m
thick with ch;~racteristicdouble layering. Lower x r o w points to brecciasof interval X and upper arrow point\ top;inilona
of interval Y (see Fig. 4 ) . Section 9 . ( c ) Calcitul-bidite typical of lithologies encasing hreccia beds and comrnonl!
breccia constit~lent.Scale 4cm long. Section 7. ((1) Breccia bed clasts. Wedge-shaped fractures indicated hy arl-ou ..\d
comminuted breccia material identified by arrow B suggest that clast material was probably lithified prior to RON
Section 8. ( c ) Disorganized breccia bed. Arrow point\ to individual clast. Bar 2.7m long. Section 9. ( f ) Di\org:tn&
stratified hreccia bed. Bar 25cm long. Section 9. (c.) Transition in a breccia bed from clast interval X to stratified grainr~ur
interval Y. Arrow points to scour at top of breccia interval. Scale 5 cm long. Section 8. ( 1 7 ) Inversely graded hrecci;~..Anw
points to base of hreccia bed. Bal- 25 cm long. Section 7.
KRAUSF. AND O1.I)EKSHAW
194
C A N . J . EARTH SCI. VOL. 16. 1979
TABLE
3. Reported suh
Author
-
Age
Rock unit
Location
S~ntifu
Reinhardt (1977)
L. Camb.
Camb.
Western Piedmont,
Appalachians, U.S.A.
Southeast Missouri, U.S.A.
Ye
Snyder and Ode11 (1958)
Araby and Frederick
Limestone Formations
Boneterre Formation
Field, B.C., Canada
Hot Creek Range,
Nevada, U.S.A.
Newfoundland, Canada
k
y:
-
Mcllreath (1977)
Cook and Taylor (1977)
Hubert rr (11. (1977)
-
--
~ i n t e r e and
r
Murphy
(1960)
Conaghan ct (11. ( 1 976)
--
M. Camb.
U. Camb.
L. O r c !
M. Camb.
M. Ord.
Sil.L. Dev.
L. Dev.
-
--
--
--
Dev.
- -- .
-
---
Lone Mountain Dolomite and
Roberts Mountain Forniation
Nubrigyn Formation
--
-
\'t
Roberts Mountains,
western Nevada, U.S.A.
New South Wales, Australia
Yt
~
---
Napier Formation
-
U. Dev.
Ye
.
-
--
--
Playford and Lowry
(19% .- Ilopkins (1977)
Boundary
.Limestone
Hales Limestone
--_
Cow Head Breccia
Canning Basin,
'ip
Western Australia
Rocky Mountains,
\'t
Alberta, Canada
Rocky Mountains,
Ye
Alberta, Canada
Marathon region, Texas, U.S.A. Ye
Cook et a/. ( 1972)
U. Dev.
Miette and Ancient Wall
buildups
Ancient Wall buildup
Thomson and
Thomasson (1969)
L. Penn.
Dimple Limestone
Davies ( 1977)
Penn.
Perm.
Perm.
Hare Fiord Formation
Perm.
Bell Canvon Formation
Guadalupe Mountains.
west
and NM, U.S.A
Guadalupe Mountains,
YE
west Texas and NM, U.S.A. _
Guadalupe Mountains,
la
west Texas, and NM, U S A .
Othris, Greece
Ys
Thomson and
Thomasson ( 1962)
Newell et (11. (1 953)
-
Kriz Lens
Sverdrup Basin,
Canadian Arctic
Sierra Diablo. Texas. U 3 . 4 .
exa as
'
!r
--
Jacka et a/. (I 968)
Perm.
Delaware Mountain Group
Dunham (1972)
Perm.
Bell Canyon Formation
Price (1 977)
U. Trias.
L. Cret. L. Jur.
Gavriana and Meterizia
Formations
Lower Breccia Formation
Finger (1975)
M. Jur.
Garrison and Fischer
(1969)
Schlager and Schlager
( 1970)
Remane (I 970)
Jur.
Baldomero Carrasco
(1977)
Enos (1977)
Cret.
French Pre-Alps
YE
Bardella Formation
Graubunden, Switzerland
la
Adnet Bed and
Ruhpolding Radiolarite
Tauglboden Schichten
Northern Limestone, Alps,
Austria
Eastern Alps, Austria
Vocontian Trough
Subalpine ranges,
southeastern France
Ye
Van Hoorn (1969)
U. Cret.
Campo Breccia Formation
Sierra Madre, Oriental,
Mexico
Tampico Embayment,
Mexico
Southcentral Pyrenees, Spain
?a
Cret.
Upper TamaulipasTamabra Limestone
Tamabra Limestone
Pszczolkowski (1978)
U. Cret.L. Eoc.
Paleoc.- Eoc.
Buena Vista and Ancon
Formations
Paraguito Boulder Bed
Cordillera de Giianigiianico.
western C
Lara, Venez
Hendry (1972)
- .- -
U. Jur.
U. Jur.L. Cret.
"
IE
-.
Renz et a/. (1955)
NOTES:Descriptions and figures in these publications indicate that many breccia have a capping turbidife comp
\e
Is'
'
\a'
KRAUSF. AND OLDERSHAW
195
rrcnccs
Bed type
Coninients
rg:~nireilpredominate.;. Also
rtlcd 2nd inverscly_gradcd -
Some bed.; anialganiated. Individual bed thickness variable
- -
~
~
-
-
-~ --
renni~cd
--rp;~nizedand normally graded,
lh typeswith stratified~aps rp:~ni/ctl and normally graded
-
-
-
-
-
-
-
- -
--
-
-
- --
--
~
- --
- - -- -
-
-
-
-
-
--
-
-
-
-
-
- -
- --
-
I other
mJel L*
hre
-
-
-
-
-
-
- - --
-
-
-
- -
-
~
-
-
--
-
-
-
-
-
-
-
-
-
-
--
-
-
-
--
-
-
- -
-
-
-
~
-
-
-
-
-
--
Clast-supported framework. Surround~nglithologies
dCo?llcd and =ured
Some hctls capped hy calcarcnitci which may he
disconfinuous
Individual beds variable in rhicLnc\s. Channeling apparent
-
--
--
~
~tnglydisorganized, locally
;cr\cly graded
--ongraded
(disorganized?). Partially
arxlcd (disorganized'?). Graded,
:d b v ~ a r t i a ! B ~ u m ?Sequence
mized
-
,v 3'1U
-
Some hcd\ may amalganiate. Internal shear surfaces and
scourcd haws
- - Individual beds variahlc in thickness. hlud-supported clasts
2nd cla\t-supported framework with grainstone matrix
Shect and lense-like deposits
-
minantly disorganizcd
.
-.
Some bcds with only slope-derived clscts
n;~llygraded
rganizcd prcdoniinates. Also
~tledwith stratified~cap
rgani~cd
-
-
~
~
- -
-
Tongue shaped in plan view. Individual hcd thickness
variahle. Anialganiation of sonic heds. Commercial lead--- 7inc niincraliz:~tiqn-in-mgrix of- eve^-nl_heds
1
-
-
.-
-- -
~
-
-
- -
-
--
-
-
-
- -
-
-
- -
-
-
-
-
-
-
-
-
- -
-
-
-
-
-
-
- -
-
Large trough cross-stratification in some hcds
_
-p
-
-
-
--
-
-
-
~
-
-
- -
-
-
--
-
Individual hed thichncss vari:~hle,Icnticular, and tiioundlikejn-prpfile. -Sheetllihc a l i l n q s ~ r f i e t ~ ~ c ~ I J X h n i
q
Mud content i n c r c a w to\*.ards hasc of flow. Abrupt
change in thickness from 40:1.5 m in 4.50 m
Indi\idual bed variahle in thickness. Hlunt-cdged tcrniinus
mired and graded
in one slide in Radcr menihcr. Disorgani~edto graded
transition
over- 5 km
Individual heds vnriahle in thicknc.;.;. Downcurrent change
Songr;ded (disorganized'?). Normally
-in-clasts-from-- chaotic to aligned
and i nverscly graded
Ihorg;mired('!) to graded(?). Units
Clasts indurated prior to transport. Grading better
ices
developedin ba$i~a~dcpo$s
c:ippciIJ>y_t_ur&l
I'rcdon iinantly graded, inverse and
C'hange into overlying turhidite gradual. Double layer
normlally
develops as flow nioves downslope
linyracled (disorganized?) and graded.
Thickness of individual hell variable. Large tabular and
Strat ]lied-caps-- - - --- trough
cross-stratification
on graded brekias
-.
-GradedI hreccia, also disorganized with
Individual hed thickness variahle
clasts nintr iz-supported
.
--Slunip-ruhhle bed (disorganized?). Also
Figure I0 disaggregation of a slump into rubble along
grad(:dp
hinges-of slumpfolds
p---.
?
Beds tongue-shaped in outline. Grain sire of breccia material
decreases away froni source. Reds may amalgamate
Early induration of clasts indicated by wedge-shaped cracks
l)iwrg:mized matrix-supported.
I)iw rganired with matrix and clast
infilled with spar and mud. Flows niay show amalgamation.
lndividual bed thickness variable over short distances.
cupptorted framework and calcarenite
Figure 10 illustrates association between underlying disorcaps. Graded with calcarenite caps:
hreccia
and-overlying
('n~~crobreccia')
-ahrulpt or gradual transitions
-ganized
- turbidite
- -- --Diwrg;mized. Normally graded with
Matrix and clast supported frameworks
.
UVCI lyingturbidite
-,?
Oil production from breccia beds in Poza Rica Trend.
Matrix and clast supported frameworks
raded (disorganized?), graded and
Downslope gradation of breccia types from ungraded
aded with transition t o Bouma
t o graded
tcrval
-mally graded, possibly capped by
r@d?es
Jed
.
-
-~
-
--
-
-
-
-~
--
--
- --
-
-
-
-
- -
-
-
-
-
-- -
-
-
-
-
-
---
-
- ---
-
-
-
-- - -
~
- -
-
-
- - -
-
not necessarily the opinion1 o f the listed authors.
--
-
--
- -
- - - - --
-
--
-
I?
-
-
-
-
--
-
-
- - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- -
-
-
- --
- - - -
-- - -
~ w r o u nm a
-
-
-
---
---
-
- -
-
-
-
- -
-
-
~
-
--
-
-
-
-
-
-
- -
196
C A N . J . EARTH SCI. VOL. 16. 1979
posits within and below some breccia be& oftb
Sekwi Formation would develop during t r - i t n y v
as the overlying turbidity current scoured thetcyv
the underlying flow, and as portions of the rurbi,
cloud spilled over the sides of the main bod! offloe
to scout-the surrounding slope and plain sedimen:.
Ultimately. grainstones in scoured pocket\ \ \ o i
be trapped by oncoming and expanding curp, P
the denser flow.
FIG. 4. Idealized hr-eccia hed with a lower breccia interval ( X )
and an upper grainstone turhidite interval ( Y ) . Interval X is
cha~.acterizedhy: (1) large clmts commonly grain suppol-ted
and. less frequently so. matrix supported; (2) matrix o f clay o r
- fine-fr;~gmentetlclnst material: (3) clasts ungraded or di\playing
coarse tail and composition grading: (4) cliists aligned. imhricated. or completely disorganized; ( 5 ) shear zones at top and
base o f some flows (XI and XIII): (6) a S C O L I ~ C L ~base 01-rarely
amalgamation with underlying material: also. beds heneath
hrecc~amay he deformed; and (7) channel-like deposits of
grainstone material fi-om interval Y within and along ha\e ofhed.
Intel-val Y is ch:~lxcterizedhy grainstones in following downw a d prop~tssingsequence: ( I ) ripple-drift cross laminations
(Y,,,): ( 2 ) plane parallel laminations (Y,,): (3) massive poorly
graded or ungraded (Y,): and (4) ahrupt size tramition into
undel-lying breccia and (or) well-developed scour surface.
by reverse shear to a point where it was abruptly
thrown into turbulent suspension. Hampton (1972.
pp. 785-788, Figs. 12, 13)suggests that the pressure
distribution around the front of a debris flow explains the mixing mechanism. In the urea of reverse
shear, near the front edge of the flow, pressure is
large enough to hold material to the debris flow
surface. A low pressure zone exists near the nose of
the flow behind the layer of reverse shear, where
the flow direction of fluid along the surface of the
debris flow is opposite to that in the layer of reverse
shear. Material in the layer of reverse shear moves
continuously into the region of low pressure. where
it is lifted awav from the debris flow surface into the
over-lyingturbulent region. The cloud that develops
in this mixing process is a turbidity current.
The two-layer Sekwi beds can be explained in
terms of Hampton's experiments. Caps on breccia
beds would be the result of a turbidity current that
developed on top of a moving carbonate mass as
material along the front of the flow was sheared
backwards into the low pressure zone and lifted
into the overlying turbulent region. Grainstone de-
A Depositional Model for Submarine Carbonate
Rreccias
Breccia bed occurrences in the Sekwi Form-.
tion, identified in a traverse across regional driL
(Fig. 2). indicate that single-layer breccia lied^^:
concentrated in upper parts of sections mea\ui:;
and that both normally and inversely to norm-?
graded breccia beds are concentrated in the l o n ~
Breccia beds may be arranged in a depo\itin
sequence which suggests a downslope gl.;~iI;~tio~
breccia bed types (Figs. 6.7).
The. Model
With increasing transport there is a donn\!oy
transition from slumped deposits. to discqxinizr:
flows, to normal and inverse-normal graded Rw
Sediment masses generated during slump r e 4 ment further downslope and leave the follw.;
sequence: ( 1 ) tlisor.grlni:c~dhetl: breccia cl;i\t\::.
disordered and matrix or grain supported; rllLl.orgtini;.ccl strcrt$ietf bc~ef:with matrix or grain \up
ported breccia clasts overlain by massive. p;ti;t~s
laminated grainstones of comminuted bit;material; and (3) grrrdecl (norn7c11or ill:!-':
strrrt(fifi~(1k r d : normal to invel-se-normi c::
supported clasts are overlain by a grainstone t u ~
dite of comminuted breccia material.
It is suggested here. that breccia bed\ ;lr:,?
itiated from slumps. a concept also advoc.;~rell~
Hopkins (1977). Cook (1978), Abbate pt ( I / . (197~~~
and others. It is also suggested hew ti
downslope movement of large sediment cr:iiil
flows. initiated from slumps, may occur in lmtim
other than submarine canyons and feederchmh
a view which is supported by the occurrensed
laterally extensive breccia beds in the Sek\\iFs
mation. and by previously published workonbrccia beds with similar characteristics (Hopkin~lT
Cook and Taylor 1977; Hubert et d.1977: ,Mw
et (11. 1970). in addition to recently publkhetidcounts of very large slumps on modern continrrrri
margins (e.g., Embley 1976; Dingle 1977:\ ! W d
trl. 1976).
Conclusions
m
Carbonate breccia beds of the
IDlSTURBED WATER
RBULENT
I
I
/
I
CLOUD
INCREASED PORE PRESSURE ?
FLOW D l RECTIOW
<
'
FII. l)t.~:t~.~ni
~llustr;~tcs
some of the olwrvationh made bv H m p t o n (1972) dul-inp flume studies with suh;~queousclay
brrtc\.Htc;ii:~ I ~ l may
s develop two diqtinct zones (see Fig. 4) a s a result o f turbidity current.; that developon t o p o f the
ruinrn,w~~I
liic \ctlinic'nt gravity flow.
SUBMARINE BRECCIA BED SEQUENCES
DISORGANIZED
1
STRATIFIED
DISORGANIZED
2
STRATIFIED,
NORMALLY
GRADED
STRATIFIED,
INVERSELY
TO NORMALLY
GRADED
3
6
-
UGGESTED RELATIVE POSITIONS DOWNSLOPE
TI(, 6 . Ihccc~,~
lhcd typej ohserved in the field: (I)disorganized bed-breccia clast\ arc disordered and matrix or grain
uf(x~~tcd:~l~~l~\t~~-r;~~ii/cll.
.;tr;~tificd bed-disordered hreccia clast\. matrix or g a i n supported, and overlain by massive
&ptr.!llcl I.mn:~rcdsr:rinstone\ of Sr:~gmentedbreccia material; and ( 3 ) nol-mully 01- inversely-normally graded. stra~ified
k J d ~ t i . ~~I,I\I\
t . ~ NC' nol-m;~llyor inversely to normally graded. matrix consists predominantly o f comminuted breccia
.lur~,~l.;~nd
IhI\ ;~rcc:~ppedhy a turbidite with Houma A. R . C interv:~l\.
t rubm;~rinc.d i n i c n t gravity flows that reprelm rewdirntnr;~rin~i
episodes. Most o f these de-
kt?;lrcr\\u-l;~yersystems that develop during a
@clranywrlwent. The deposit may consist of a
aciml intcrvd with or without a capping turbik in~crul. 'I'l;~nsport and resedimentation
p l o p of Ivcccia beds may take place any-
where on the continental slope and rise. and i s not
restricted to submarine canyons.
Clasts i n u breccia bed are commonly grain supported, pebble to boulder sized calcareous wackestones and mudstones. Each breccia bed i s
characterized by one o f the following clast fabrics:
disorganized (types 1 . 2). normally graded. or in-
CAN. J . E A R T H SCI. VOL. 16. 1979
SLIDE SCAR
REMOLDED AND
DISORGANIZED
FIG.7. Suggested suhmwine sediment gravity flow model for
carbonate breccia bed deposits observed in the field. 0hserv:ttion5 of subaerial debri\ flows (Johnwn 1970). published examples of carbonate hl-eccia heds. and examination of sections
t h l - o ~ ~ breccia
gh
heds of the Sekwi Formation indicate the probable geometry of the\e depo\it\ (Newell 1.r (11. 1953: Snyder and
W e l l 1958: Thomson and Thomarison 1962: Schlagcr and
Schlager 1970: Cook rt trl. 1972: PI:tyford 1977). B~.ecciabeds
are irregular in \hnpe. sheet and tongue-shaped in plan view.
lenticular in frontal section. and possess pinch and swell topography in longitudin;tl section. The pl-oce.;s is initiated by slumping and sliding of slope deposits (calcit~rrbidites).M:~terialflows
:I\ a liquefied slurry. disaggregates. and remolds: a deposit of
disoriented clasts is produced. Continued downslope movement
of the undmined sediment gravity flow and mixing with the
overlying water mass produces a tul-hidity current on top of the
flow: the depwits comist of :I basal clast interval with a turhidite
on top and a depositional sequence a s wggested for Fig. 6. The
t~lrbiditycurrent racesaway from the basal portionofthe flowas
the latter comes to u stop. A turhidite lacking a breccia bed
component will blanket the surrounding sea floor.
versely to normally graded (type 3 ) . Textural
characteristics of these deposits appear to be a
function of transport distance. With increasing
transport there is a downslope transition from
slumped deposits, to disorganized. to normal and
inverse-normal graded deposits.
Acknowledgements
We extend our sincere thanks to Dm. A. J . Rowell, G . V. Middleton. J . C. Hopkins, M. P. Cecile,
D. W. Morrow, and Mr. F. A. Stoakesforcritically
reading the manuscript and offering useful suggestions for its improvement.
We are grateful to MI-.C. Lord, Nahanni District
Geologist, Department of Indian Affairs and
Northern Development, for his support in and
away from the field. We would also like to thank
Messrs. H. Speelrnan and N. Webber, our amiable
and hard-working field assistants. Field work on
the Sekwi Formation was sponsored by the Department of Indian Affairs and Northern Develop-
ment. F. Kmuse gratefully acknodcdgei ll
financial support of the Institute Venezol;~noJr
Investigaciones Cientificas.
This paper constitutes part of the on_coin:h
sertation research of the senior author.
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